Modern electronic equipment such as televisions, telephones, radios and computers are generally constructed of solid state devices. Solid state devices are preferred in electronic equipment because they are extremely small and relatively inexpensive. Additionally, solid state devices are very reliable because they have no moving parts, but are based on the movement of charge carriers.
Solid state devices may be transistors, capacitors, resistors, and other semiconductor devices. Typically, such devices are fabricated on a substrate and interconnected to form memory arrays, logic structures, timers and other integrated circuits. One type of memory array is a dynamic random access memory (DRAM) in which memory cells retain information only temporarily and are refreshed at periodic intervals. Despite this limitation, DRAMs are widely used because they provide low cost per bit of memory, high device density, and feasibility of use.
DRAMs typically include an array of memory cells accessed by a series of word lines and bit lines. Each memory cell includes an access transistor coupled to a storage capacitor. The access transistor is formed from a portion of a word line disposed over a channel that is defined in an underlying substrate. A source and drain for the access transistor are also defined in the substrate. The source is shared with an adjacent access transistor and connected to a bit line. The drain is connected to the storage node.
Efforts to increase DRAM density have concentrated on minimizing the planar area of the memory cells. The planar area of the cells, however, is constrained by the configuration of the access transistor, the storage node, the word line, and the bit line.